PHYLUM ECHINODERMATA

Definition  and Indroduction

Bilateral and deuterostomial  eucoelomate  eumetazoa  with rough , tough and  leathery skin  due  to a dermal skeleton of usually  spiny  ,calcareous  ossicles, and  the  basic  bilateral symmetry of larva modified in the adult, to a pentamerous radial   symmetry .

The terms Echinodermata  means  spiny  skin (Gr. Echinos= spiny + dermatos =skin) About 6,000 living  and 20,000 extinct  species known.

Brief   History

Although Jacob klein (1738) had  earlier coined the  name  Echinoermata  yet Linnaeus  included  these  animals  under Mollusca  and  Lamarck under his  class  radiate  as echinoderms .Finally  leuckart (1847)  raised  the group  to  the status  of a  separate  phylum.

Important Characters

1.    All  species marine  sessile, creeping or  crawling  at sea bottom,. Rarely free swimming .

2.    Body mostly of moderate size, spherical, cylindrical  or star shaped triploblastic  and unsegmented .

3.    Symmetry  bilateral  in larvae  but pentamerous radial  in adults .

4.    No  distinct head  or bagin .

5.    Oral surface  distinguished  into 5 radial ambulacral areas alternating  with  interambulacral  areas  around  an oral aboral axis .

6.    Body wall  rough, tough  and leathery due  to mesodermal calcareous  ossicles  and spins ( internal skeleton) covered  by epidermis  which  is usually  ciliated .

7.    Body cavity  is a true  enterocoelic  coelom  surrounded by  ciliated  peritoneum ,and divided  into  several tubular spaces and sinuses. The  latter from 3 specialized system ,viz water  vadcular, haemal and perihaemal system.

8.    Locomotion  by special small  and hollow, sac like  tubular  processes , called  tubefeet,  and connected with  water  vascular system .Tubefeet  also  assist in feeding  sensory reception  and respiration .

9.    Haemal  and perihaemal system  are  connected with  blood  circulation.

10. Digestive  system  simple. Alimentary  canal  usually  coiled. No anus or  intestine in  some.

11. No  organs for excretion. Certain  respiratory  organs may  be present .

12. A diffused  and uncentralized  nervous system and less developed sensory  organs.

13. Reproduction mainly  sexual  sexes separate  extensive  power of  regeneration .

14. Fertilization  external in water. Life  cycle  includes bilateral, ciliated  larval stages  which  metamorphose into  adults.